1. Field of the Invention
The disclosed methods and systems relate generally to electrostatic discharge protection, and more particularly to protecting a sensor input from electrostatic discharge.
2. Background Information
By design, or often as a secondary effect of semiconductor manufacturing technology, inputs of integrated circuits (“ICs”) may become highly conductive in the manner of a diode if the voltage on the input is below the most negative potential on the IC or above the most positive potential. This potential problem may be avoided by specifying that inputs must not be more than 0.3 volt (“V”) above the positive potential or more than 0.3 V below the negative potential. In most cases, no harm is done and, by specification it is often allowable to exceed these limits if the resulting current is limited to 10 milliamperes (“ma”) or less. Latch-up or damaging effects typically are likely in the 100 ma vicinity, although these levels are usually not published. The allowable current levels for amplifiers with very low bias currents may be less than these levels, but these allowable current levels have likely increased as technology improves. However, the ability of ICs to accept surge currents is typically very limited. Clamping the input signals with diodes to the supply voltages and inserting some resistance between the diode clamps and the device input may provide protection, but there are three critical issues: (i) the magnitude of the potentially damaging transient; (ii) the capability of the diode to carry this transient; and, (iii) the leakage of the diode and its impact on the circuit under normal operating conditions.
Some equipment for electrostatic discharge (“ESD”) provides impedances of 1500 ohms or 330 ohms. Accordingly, amperes of current will flow, though briefly, in these discharges. High conductivity diodes are available that may readily accept these discharge currents. However, even if described as “low leakage,” these leakages are likely to be orders of magnitude worse than the bias currents of ICs with low input currents, e.g., nanoamperes versus femtoamperes, or worse.